The Trump administration’s proposal to eliminate funding for a program that addresses major environmental and health threats in the Great Lakes would have a devastating impact on millions of Canadians, officials and environmental groups said Thursday.

The White House’s 2018 budget proposal would, if approved by Congress, gut the Great Lakes Restoration Initiative, a federal program that has helped to remove water pollution and harmful algae on both sides of the border. The issue could become one more point of friction between the United States and Canada, already divided over trade and immigration.

The budget proposal, which reflects Trump’s wish to drastically reduce the size and scope of the Environmental Protection Agency, has dismayed the Canadian government, which cooperates with the United States to clean up and protect the Great Lakes. The lakes are the source of drinking water for 45 million people — including 10 million in Ontario, about 90 percent of its population. Ontario has the largest shoreline on the Great Lakes of any jurisdiction in Canada and the United States.

“Canada has a long history of working collaboratively with the U.S. and invests significant resources in restoring and protecting the Great Lakes,” Catherine McKenna, Canada’s environment and climate change minister, said Thursday. “We have done so to protect the health and economies of communities around the Great Lakes. We must now pursue that commitment to keep protecting this precious resource.”

McKenna was in Washington on Thursday to press her U.S. counterparts to maintain U.S. funding for the program, about $300 million annually. Since the program was established in 2009 by President Barack Obama, more than $2.2 billion has gone to fund more than 2,000 projects across eight states. The projects have been aimed at removing toxic waste, restoring wildlife habitats and girding against invasive species such as the Asian carp in the vast freshwater bodies, which hold one-fifth of the world’s fresh surface water. Canada contributes more than $13 million annually for Great Lakes restoration, government officials said.

Although the budget cuts would target projects in the United States, Canadian towns and cities that rely on the lake system would also be affected, advocates say. These include five binational Areas of Concern, like the Detroit River, a long-polluted waterway between Ontario and Michigan, as well as nutrient management projects in Lake Erie that focus on preventing algae blooms triggered by industrial contaminants.

Since word of the proposed cuts to the program leaked this month, dozens of Canadian mayors and other officials have spoken out about the harm their cities and towns would suffer if the proposal were to go into effect.

“As the largest city on Lake Huron, the cuts will undo decades of work by many to clean up the Great Lakes on both sides of the border,” said Mayor Mike Bradley of Sarnia, Ontario. “What good is it having lower taxes when you can’t drink the water?”

Sarnia, at the mouth of the St. Clair River, across from Michigan, began working to remove water contamination in 1985, when the city discovered a 200-foot-long oily slick beneath the river that turned out to be 2,900 gallons of perchloroethylene, a chemical used for dry cleaning that had been spilled by a nearby Dow Chemical plant. Despite marked progress in cleaning up the river and surrounding habitats, the city remains designated as an Area of Concern by the Canadian government.

Residents and politicians on both sides of the border say they are mystified by the proposed budget cuts, which they say would do massive damage to waters still reeling from decades of industrial contamination.

A large amount of the program’s funding has in past years gone toward preventing toxic algae blooms, like one in 2014 that forced Toledo, Ohio, to temporarily shut off its water intake system after samples were found to contain microcystin, which can cause liver and kidney damage and respiratory failure. Earlier this month, the Ontario and Canadian governments released a draft plan to reduce algae blooms in Lake Erie, part of the Great Lakes Water Quality Agreement with the United States.

Mark Mattsen, president of Lake Ontario Waterkeeper, an environmental advocacy group based in Toronto, said the proposed elimination of the Great Lakes Restoration Initiative would threaten the region’s $7 billion fishery. The cuts would end infrastructure plans to prevent the spread of the Asian carp, an invasive species brought to the United States 40 years ago to eat algae. It has destroyed numerous fisheries and is making its way toward the Great Lakes.

“We’re at a crossroads,” Mattsen said. “Canada is going to have to lead by example and show the U.S. this isn’t the right direction.”